# gammaStat

##### Gamma-Statistic of Pybus and Harvey

This function computes the gamma-statistic which summarizes the information contained in the inter-node intervals of a phylogeny. It is assumed that the tree is ultrametric. Note that the function does not check that the tree is effectively ultrametric, so if it is not, the returned result may not be meaningful.

- Keywords
- univar

##### Usage

`gammaStat(phy)`

##### Arguments

- phy
an object of class

`"phylo"`

.

##### Details

The gamma-statistic is a summary of the information contained in the
inter-node intervals of a phylogeny; it follows, under the assumption
that the clade diversified with constant rates, a normal distribution
with mean zero and standard-deviation unity (Pybus and Harvey
2000). Thus, the null hypothesis that the clade diversified with
constant rates may be tested with ```
2*(1 -
pnorm(abs(gammaStat(phy))))
```

for a two-tailed test, or ```
1 -
pnorm(abs(gammaStat(phy)))
```

for a one-tailed test, both returning
the corresponding P-value.

##### Value

a numeric vector of length one.

##### References

Pybus, O. G. and Harvey, P. H. (2000) Testing macro-evolutionary
models using incomplete molecular phylogenies. *Proceedings of
the Royal Society of London. Series B. Biological Sciences*,
**267**, 2267--2272.

##### See Also

*Documentation reproduced from package ape, version 5.3, License: GPL (>= 2)*